The present invention relates to polar loop transmitters which have particular, but not exclusive, application in a VHF single-sideband (SSB) radio system.
For single-sideband transmissions with an audio bandwidth of typically 300 Hz to 3.3 kHz, it has been proposed to use channels spaced at 5 kHz, thus leaving approximately 1.5 kHz as a guard band. Of particular importance in such a narrow channel system is the level of spurious, out-of-channel, transmitter emissions. In a conventional SSB transmitter, the majority of adjacent channel interference is caused by intermodulation products produced by non-linearities in the power amplifier which is provided to reproduce a low level input signal at a high level. Because linear power amplifiers are very difficult to implement at R.F. frequencies, it is preferred not to use them.
The polar loop principle is well known and has been proposed by Petrovic, V. and Gosling, W., "A Radically New Approach to SSB Transmitter Design", I.E.E. Conference on Radio Transmitters and Modulation Techniques, March 1980, No. 1980/40 page 110, as a means of producing very clean signals at high power levels. In a polar loop transmitter, an audio input signal is mixed with a local oscillator signal in a balanced mixer and either the upper or lower sideband is selected using a sideband filter. The selected sideband signal is resolved into polar components (phase and amplitude) by a first limiter and a first amplitude detector.
A voltage controlled oscillator is provided to generate a signal at the transmitter output frequency. This signal is buffered and fed to an amplitude modulator whose output is fed to an R.F. power amplifier and then to a load, for example an antenna, via a low pass filter.
An important feature of a polar loop transmitter is that it compares the high level output with the low level input to see if there are any errors and if there are, the errors can be used to effect corrections in the amplitude modulator. In order to carry out this comparison, the signal from the low pass filter is sampled and mixed down to the pilot frequency of the selected sideband. This mixed down signal is resolved in polar components (phase and amplitude) by a second limiter and a second amplitude detector. The phase signals from the first and second limiters are compared in a phase sensitive detector and the amplified and filtered output of the phase sensitive detector is applied to the voltage controlled oscillator to lock its phase to that of the input signal.
The signal envelope or amplitude signals from the first and second amplitude detectors are applied to a differential amplifier which produces a control input voltage which amplitude modulates the R.F. carrier from the voltage controlled oscillator. The setting of the D.C. component of the control input voltage is critical because, unless it is correct, the low power signal will not be copied accurately at high power. This is of special significance when the R.F. envelope is to be reduced by, for example, -70 dB with respect to peak envelope power at zero crossings of the input waveform.
A drawback to this known circuit is that, unless the amplitude detectors are matched, non-linearities in the signal paths to the inputs of the differential amplifiers will cause distortions which can lead to spurious, out-of-channel transmitter emissions. One way of obtaining a match between the amplitude detector is to integrate them on a single chip. This has been done experimentally.